1. Field of The Invention
The present invention relates to an exposure method and an exposure apparatus and, more particularly, to an exposure method and an exposure apparatus for forming an image of a pattern of a mask on an uppermost layer of a substrate by aligning a pattern formed on a given layer of the substrate with a pattern on the mask.
2. Description of the Related Art
In a method for forming plural patterns in layers superimposed on a substrate by exposure to light, an upper-most layer of the layers superimposed on the substrate is generally formed with a new pattern by aligning the new pattern with one of the previous patterns already formed in the layers on the substrate.
For comparative purposes, an example of conventional exposure methods will be described with reference to FIG. 5 illustrating such a conventional exposure method, a first step of which comprises a selection of operation steps, an exposure apparatus and an exposure reticle as a preparation step for operations. Then, a substrate is aligned under alignment conditions and subjected to test exposure to light (pilot or pre-processing), followed by a measurement of an alignment residual error of the substrate exposed to light by a given measurement device. As a result of the test exposure, when it is found that the alignment residual error for the substrate is within a given reference value, a main exposure is carried out. On the other hand, if the alignment residual error for the substrate is found to be outside the given reference value, the alignment conditions are changed on the basis of the alignment residual error measured and the substrate is aligned again under the renewed alignment conditions, followed by the implementation of a main exposure step or the re-implementation of a test exposure step.
In the conventional process as described hereinabove, the alignment conditions may change, too, when conditions for exposing the substrate to light changes. In other words, alignment conditions may also change in each case where an operation step is changed, where a different exposure apparatus is used, where a different reticle for allowing the substrate to be exposed to light is employed, where a substrate is changed for a test exposure, where there is used a different device for exposing an alignment layer with a mark formed as a reference of alignment or where a reticle used for exposure of the alignment layer is changed. In order to sustain a high level of accuracy in alignment, the conventional process requires the test exposure to be conducted again, whenever alignment conditions change. This results in a reduction of throughput.
In order to solve problems and disadvantages encountered with such conventional exposure methods, extensive review and studies have been made to develop an exposure method that can improve throughput of an exposure apparatus while sustaining a high level of accuracy in alignment.